1. Field of the Invention
This invention relates to the fabrication of integrated circuit structures, and in particular, to the fabrication of metal conducting lines on the surface of such structures using lift-off techniques, and the subsequent removal of the lift-off materials.
2. Description of the Prior Art
Lift-off techniques for forming desired regions of material on the surface of integrated circuit structures are generally known. A survey of various lift-off processes is presented in "Lift-Off Techniques for Fine-Line Metal Patterning," by J. Frary and P. Seese, Semiconductor International, December 1981, pages 70-89.
In conventional integrated circuit fabrication technology, patterns of desired material are formed on the surface of an integrated circuit structure using etching techniques. Such processes suffer from a number of disadvantages which restrict their application, particularly in the fabrication of integrated circuit structures having exceptionally small regions. For example, to insure complete etch removal of deposited material, the pattern must always be at least slightly over-etched, leading to diminished line widths for a particular line spacing. Although this problem may be lessened by using plasma or reactive ion etching, these processes in turn create chemical contamination problems which must also be solved.
The use of lift-off processes circumvents many of these disadvantages. For example, because the integrated circuit structure itself is not subjected to plasma or reactive ion etching, radiation damage to active devices is minimized. Furthermore, lift-off processing is usable over a wide range of temperatures for patterning many materials commonly used in the fabrication of integrated circuits.
Because the lift-off material, typically a polyimide, must be firmly anchored to the underlying integrated circuit to allow accurate patterning of the lift-off material itself, it is often difficult to completely remove the lift-off material from the surface of the circuit at the end of a process sequence. This difficulty is further increased when adhesives are used for attaching the lift-off material to the surface of the circuit, and when the polyimide itself is generally insoluble. Incomplete removal of the lift-off material generally results in lower yields by creating short circuits or obscuring portions of the surface of the integrated circuit structure from subsequent process operations.